In the past, nitrogen (N) fertilizers used on grass pastures in Kentucky have largely been ammoniated phosphates in mixed fertilizers and ammonium nitrate. Due to the economic advantage of producing solid urea as opposed to ammonium nitrate, availability of urea is becoming greater and, in some areas, is the only source of solid nitrogen. If urea is not incorporated into the soil immediately after application, some of the nitrogen may be lost as ammonia gas. The loss is called volatilization. The amount of loss depends on a number of conditions. High soil and air temperatures and a moist soil which is undergoing drying are contributing factors to volatilization losses. Presence of organic residues on the soil surface and a high soil pH (6.5 or above) are also thought to increase the expected loss. Based on these conditions we would expect N losses from top dressed urea to be greater when applied to pastures than when applied to row crops and the rate might be different at different times during the season.
Murdock, Lloyd W., "Comparative Effectiveness of Urea, Ammonium Nitrate, and Urea Ammonium Polyphosphate on Fescue Production" (1982). Agronomy Notes. 86.